Borchardt said the inspections revealed that "some equipment, mainly pumps, were not operable when tested or lacked acceptance criteria. Some equipment was actually missing from the locations or dedicated to other activities, and in some cases, plant modifications had rendered the strategies that were developed at the time unworkable." But, he added, "we didn't find any cases where ultimately the [safety] function could not be performed."
Miller said the Fukushima disaster has focused the task force on the impacts of multiple threats to nuclear plants, such as a major fire and an earthquake, or an earthquake and flooding, that could knock out power for a plant's emergency equipment. Backup battery power for crucial safety equipment is only adequate for four to eight hours of emergency use at U.S. plants, Miller noted. The Fukushima accident forces a re-evaluation of whether that is enough protection, he concluded.
The current NRC regulations, called the station blackout rule, do not consider the impact of disasters that could cut off both on-site and off-site power simultaneously, Miller said.
New look at generators and vents
"As a result, the regulatory guidance assumed that the event causing the off-site power disturbance does not impact on-site power sources," he said. "And therefore the availability of on-site power is based on historic reliability of emergency diesel generators to start and perform their safety function when called upon."
The four- and eight-hour battery backup capabilities assume that outside power can be promptly restored. "In the case of an extreme external event, it might take days to restore [outside] AC power, as was the case at Fukushima," Miller said.
Miller also raised the issue of the vents that lead out of the wetwell, or torus, at the bottom of the Mark I reactors at Fukushima. When reactor fuel assemblies melted during the accident, hydrogen gas built up at high pressures inside the torus. Instead of exiting the reactor containment structure, hydrogen is presumed to have leaked from the venting system and exploded.
After the Three Mile Island accident, U.S. Mark I owners were required to install hardened exhaust vents, Miller noted. But the regulation did not specify a design for the vent configurations, nor did the regulations require a specific inspection program for the vents, he said. The designs in the United States vary, he added, and there are differences in the number and location of valves that might have to be operated in an emergency situation.
The venting systems "were not specifically designed for operation during a long-term station blackout," he added. "Therefore, depending on a plant-specific design, it may be a challenge to open the vent path in a scenario like the Fukushima accident."
He summed up, "in evaluating potential new [safety] requirements, the [NRC] staff tends to lean more toward the quantitative cost-benefit aspects of the regulatory analysis rather than the qualitative defense-in-depth consideration. This can result in more weight to protection strategies and less balanced approach to defense in depth."
An NRC requirement that plant owners make major modifications to reflect new safety standards is called the "back-fit" rule, and it includes a balancing of safety benefits and regulatory costs.
Commissioner Kristine Svinicki said Miller's discussion suggested that "you could quickly find yourself swimming in the waters of back-fit. ... That is something the task force is having to confront."
"We are confronting it," Miller said. "It is an awkward time."
Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500